Accurate knowledge of operating altitude is so critical to aircraft use and safety that it is common practice in many aircraft of either the military or the civilian types to include more than a single altitude indicating instrument in the airframe. Moreover, in an extension of this redundancy philosophy, it is often deemed wise to have these parallel instruments be of differing operating principle in order that a failure event or an accuracy jeopardizing event in one instrument can be recognized and tolerated with minimum impact on flight operation.
Thoughts of this type have resulted in the inclusion of an atmospheric pressure responsive standby auxiliary altimeter instrument in presently used military aircraft including the F-15E aircraft. This instrument is usually mounted in the instrument panel of the aircraft where it provides a visually discernable indication of altitude to the aircraft pilot or crewmembers and also can also provide an electrical signal representative of measured altitude to an automatic pilot or a computer or similar apparatus. Altimeters of this type, although simple and reliable in their atmospheric pressure responsive operating principle, are found to need periodic testing, calibration, maintenance and overhaul in order to function with needed accuracy and dependability. Work of this type may be accomplished with aid of the present invention while the altimeter is yet within the instrument panel of the aircraft or may be accomplished while the instrument is removed from the aircraft and undergoing bench examination or repair.
Heretofore it has been common practice to employ a collection of equipment in order to verify the satisfactory function of the atmospheric pressure responsive standby auxiliary altimeter instrument. In addition to a source of controllable pressure for changing the altitude indicated by the instrument under test, it is, for example, common to use three power supplies, two digital multimeters and a group of fourteen individual test leads in order to access the three active functions included in such altimeters. In the instance of the standby pressure altimeter instrument, for example, these functions include an electrical potentiometer controlled as to wiper arm position by the pressure responsive element of the altimeter, a mechanical vibration generating element used for reducing the effects of mechanical “stiction” in the linkage of the instrument and an assembly of electric lamps used for lighting the instrument face. In the F-15E standby pressure altimeter instrument, each of these functions happens to require a different operating voltage in order to function properly.